RTP technologies have developed to increase manufacturing throughput of semiconductor wafers while minimizing their handling. The types of wafers referred to here include those for ultra-large scale integrated circuits. RTP refers to several different processes, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN). For example, in the particular application of complementary metal-oxide-semiconductor (CMOS) gate dielectric formation by RTO or RTN, thickness, growth temperature, and uniformity of the gate dielectrics are critical parameters that influence the overall device performance and fabrication yield. Currently, CMOS devices are being made with dielectric layers that are only 60-80 angstroms (.ANG.) thick and for which uniformity must be held within a few percent. This level of uniformity requires that temperature variations across the substrate during high temperature processing cannot exceed a few degrees Celsius (.degree.C.).
The wafer itself often cannot tolerate even small temperature differentials during high temperature processing. If the temperature difference is allowed to rise above 1-2.degree. C./cm at 1200 .degree. C., the resulting stress is likely to cause slip in the silicon crystal. The resulting slip planes will destroy any devices through which they pass. To achieve that level of temperature uniformity, reliable real-time, multi-point temperature measurements for closed-loop temperature control are necessary.
Optical pyrometry is being widely used for measuring temperatures in RTP systems. Pyrometry exploits a general property of objects, namely, that objects emit radiation with a particular spectral content and intensity that is characteristic of their temperature. Thus, by measuring the emitted radiation, the object's temperature can be determined.
A pyrometer measures the emitted radiation intensity and performs the appropriate conversion to obtain temperature (T). The relationship between spectral emitted intensity and temperature depends on the spectral emissivity of the substrate and the ideal blackbody radiation-temperature relationship, given by Planck's law: ##EQU1## where C.sub.1 and C.sub.2 are known constants, .lambda. is the radiation wavelength of interest, and T is the substrate temperature measured in Kelvins. According to an approximation known as Wein's distribution law, this expression can be rewritten as follows: ##EQU2## where K(.lambda.)=2C.sub.1 /.lambda..sub.5. This is a good approximation for temperatures below about 2700.degree. C.
The spectral emissivity .epsilon.(.lambda.,T) of an object is the ratio of its emitted spectral intensity I(.lambda.,T) to that of a blackbody at the same temperature I.sub.b (.lambda.,T). That is, ##EQU3## Since C.sub.1 and C.sub.2 are known constants, under ideal conditions, the temperature of the wafer can be accurately determined if .epsilon.(.lambda.,T) is known.
However, despite its widespread use in the semiconductor industry, optical pyrometry still suffers from limitations due to an inability to accurately measure the emissivity of the substrate. Moreover, even if the emissivity of the substrate is known at a given temperature, it changes with temperature. The changes are usually not accurately measurable, and thus they introduce an unknown error into the temperature measurements. Errors on the order of 10.degree. C. or more are not uncommon.
The spectral emissivity of a substrate depends on many factors, including the characteristics of the wafer itself (e.g. temperature, surface roughness, doping level of various impurities, material composition and thickness of surface layers), the characteristics of the process chamber, and the process history of the wafer. Therefore, a priori estimation of substrate emissivity cannot provide a general purpose pyrometric temperature measurement capability.
Techniques for reducing the effect of changes in wafer emissivity on temperature measurements are known. Some techniques are disclosed in U.S. patent applications Ser. Nos. 08/359,302, entitled "A Method and Apparatus for Measuring Substrate Temperatures", to Peuse et al., filed Dec. 19, 1994, and 08/641,477, entitled "A Method and Apparatus for Measuring Substrate Temperatures", to Peuse et al., filed May 1, 1996, both of which are assigned to the assignee of the present invention and incorporated herein by reference. One such technique involves placing a thermal reflector near the back surface of a target substrate to form a reflecting cavity which causes thermal radiation from the substrate to be reflected back to the substrate. A light pipe, which is inserted through the reflector into the cavity, samples radiation from the reflecting cavity and delivers the sampled light to a pyrometer. Assuming an ideal reflector, it can be shown mathematically that because all of the thermal radiation emitted from the substrate is reflected back onto the substrate, the reflecting cavity acts like an ideal black body. That is, the intensity of the thermal radiation within the reflecting cavity will not be a function of the emissivity of the surface of the substrate. Stated differently, in the ideal case the reflecting cavity increases the effective emissivity of the substrate to a value equal to one. However, because the reflector will be less than perfect, the effective emissivity of the substrate will be higher than the emissivity of the wafer but less than one.
One instance in which a reflector degrades to less-than-perfect reflectivity arises during chemical vapor deposition (CVD). In CVD, chemically reacting species may deposit on the reflector surfaces surrounding the pyrometers and, perhaps even more problematically, on the pyrometers themselves.
In particular, gaseous products of the chemical reactions on the wafer are expected to be exhausted. However, some amount of these gases may undesirably flow to regions below the plane of the wafer. For example, a typical silicon deposition may occur by the reaction of trichlorosilane (TCS) and molecular hydrogen (H.sub.2) in a processing region above the wafer. Occasionally, some of the process gases may leak to the region below the wafer due to imperfections in the edge ring supporting the wafer or due to incomplete coverage of the edge ring by the wafer.
The leaked gases are approximately bounded in a cylindrical region which spans the distance between the wafer and the reflector. This distance is variable. The temperature of the top boundary of this region (the wafer) is typically about 1100.degree. C. The temperature of the bottom boundary of this region (the reflector surface) may be water-cooled and at a temperature of between 50.degree. C. and 200.degree. C. such as about 150.degree. C. Under these conditions and this thermal gradient, it is often noted that trapped TCS gas is converted to silicon chloride (SiCl.sub.2) and hydrogen chloride (HCl) gases. These gases tend to form undesirable deposits on the reflector surface due to condensation. Undesirable deposits also occur on the backside of the wafer. Besides the reflector, other regions which may be so affected include the region directly below the wafer (within a well which typically contains a rotation mechanism) as well as the region surrounding the rotation mechanism. Damage and corrosion may be caused by the presence of hot gases in these regions.
One way of eliminating undesirable deposits is by use of a purge gas below the wafer. Such a purge gas may be used to direct undesired gases away from the reflector and the wafer backside. Such a system is described in U.S. Patent Application entitled "Method and Apparatus for Purging the Back Side of a Substrate During Chemical Vapor Processing", to Deaton, et al., filed on even date herewith, assigned to the assignee of the present invention and incorporated herein by reference.
Such systems do not specifically address the thermal gradient that occurs between the substrate and the reflector. Thus, it would be useful to provide a system which also reduces the effect of this thermal gradient.